The purpose of this project is to design spin label probes which can be used to study the binding of mutagens to nucleic acids. Since acridines are mutagenic and bind strongly to nucleic acids, we have designed and prepared acridine spin labels containing the stable nitroxide free radical. The binding of these acridine spin labels to nucleic acids has been studied by electron spin resonance. These spin label derivatives bind to nucleic acids by intercalation and have high base affinity. However, this affinity is not absolute. We have used these labels in studying the binding of histone H1 to DNA in the major groove.